1. Field of the Invention
This invention relates to a simple, optical, lead angle determining gun sight which is particularly adapted for antiaircraft defense.
2. Prior Art
The typical iron speed ring gun sight comprises an objective of three concentric rings spaced forwardly of an ocular ball. The sight is designed to work at a predetermined cross-over distance, e.g. 800 meters for a 20 mm gun. Each particular type of aircraft is assumed to fly in cross-over at a known, predetermined speed. The gun is bore-sighted to the center of the rings. In use, the sight is aimed so that the target aircraft lies on an appropriate speed ring, e.g. outer ring for 300 knots, middle ring for 200 knots, inner ring for 100 knots, on a flight path directed towards the center of the rings.
An improvement on the iron sight comprises a combination prism and a slightly reducing, fixed power, telescopic system. The image from the telescopic system is combined with the directly observed target in the combination prism so that the eye sees two images of the target, one of which is larger than the other. When the sight is aimed at a fixed target, the images should be concentric. The sight is aimed at a moving target so that the larger image should chase the smaller image (from the telescopic system) along a common flight path, whith a predetermined multiple of large image target lengths between them. The lead angle between the two images is a fictitious lead angle and forms only a predetermined fraction of the real lead angle. The gunner must consult a chart showing the multiple of target lengths to be used for each combination of target speed and range; e.g. for four speeds and three ranges, one out of twelve possible multiple lengths must be selected.
Other optical systems of interest are shown in: U.S. Pat. No. 1,047,051 which shows a two-field view system. U.S. Pat. No. 3,152,214 which shows a 300 m system to change the field of view. U.S. Pat. No. 2,619,874 which shows a two-field view system. U.S. Pat. No. 3,407,302 which shows a system to change optical paths. U.S. Pat. No. 2,753,760 which shows an image changing system. U.S. Pat. No. 2,578,013 which shows a drift sight.